🛡️ Sentinel: [HIGH] Fix SSRF bypass via httpx redirects

🚨 Severity: HIGH
💡 Vulnerability: The `/process-url` endpoint used `httpx.AsyncClient` with `follow_redirects=True`. While the initial user-provided URL was validated against SSRF protections (blocking private/internal IPs), the client implicitly followed subsequent HTTP redirects without validating their target locations. This allowed an attacker to bypass the initial check by supplying a valid URL that redirected to an internal IP or cloud metadata endpoint.
🎯 Impact: An attacker could potentially access internal network services or cloud metadata endpoints.
🔧 Fix: Implemented an `event_hooks` listener (`validate_redirect`) on the `httpx.AsyncClient` that intercepts responses, extracts the `Location` header, resolves the absolute target URL, and applies the same `validate_url_safety` check before allowing the redirect to be followed.
✅ Verification: Ran `pytest tests/test_url_upload.py`, formatting checks via `ruff format` and linting via `ruff check`.

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/api/url_upload.py

@@ -151,6 +151,19 @@ async def process_url(
     if not safe_filename:
         safe_filename = "download"
 
+    # Hook to validate redirects and prevent SSRF
+    async def validate_redirect(response: httpx.Response):
+        if response.is_redirect:
+            location = response.headers.get("Location")
+            if location:
+                # Resolve relative URLs
+                next_url = urllib.parse.urljoin(str(response.url), location)
+                try:
+                    validate_url_safety(next_url)
+                except HTTPException as e:
+                    # Reraise as a RequestError so httpx aborts the request
+                    raise httpx.RequestError(f"Unsafe redirect target: {e.detail}", request=response.request)
+
     # Download file with security measures
     # Initialize target_path to None to prevent UnboundLocalError in exception handlers
     # that may execute before target_path is assigned during error cases
@@ -162,6 +175,7 @@ async def process_url(
         async with httpx.AsyncClient(
             timeout=settings.http_request_timeout,
             follow_redirects=True,
+            event_hooks={"response": [validate_redirect]},
             headers={
                 "User-Agent": "DocuElevate/1.0",  # Identify ourselves
             },